1. Field of the Invention
The present invention relates generally to Polycrystalline Diamond Compact (PDC) drill bits and their methods of manufacture. More particularly, the present invention relates to methods and apparatus to improve and manufacture the internal hydraulics of a PDC drill bit. More particularly still, the present invention relates to methods and apparatus to improve the flow characteristics of drilling mud through nozzles of PDC drill bits to minimize areas of flow separation therethrough.
2. Background Art
FIG. 1 shows a typical rotary drilling rig. A drill bit 10 is connected to the end of a drill string 12. Drilling fluid, typically referred to as mud, is pumped through the drill string 12 into the drill bit 10 by surface equipment 11. The mud performs a variety of functions. For example, the mud cools the drill bit 10, cleans the cutting structures, helps penetrate the formation, and carries the cuttings to the surface. To accomplish these tasks, a high flow rate of mud must be maintained while drilling. The desired flow rate is often as high as possible based on the surface equipment 11, pressure losses in the drill string 12, and the capabilities of the equipment in the drill string 12 to handle the flow.
Drill bits used to drill wellbores through earth formations generally fall within one of two broad categories of bit structures. Drill bits in the first category are known as “roller cone” drill bits. Drill bits of this type usually include a bit body having a plurality of legs, each having at least one roller cone rotatably mounted thereto. Typically, roller cone drill bits are constructed as three-leg bits, but two leg and single leg drill bits are available. As the roller cone bit is rotated in contact with the formation, cutter elements mounted about the periphery of each roller cone roll over the bottom hole formation, scraping and pulverizing the formation into small pieces that are carried to the surface with the returning annular fluid. An example of a prior art roller cone bit is shown in FIG. 2. The roller cone bit includes a bit body 215 having at least one roller cone 214 rotatably mounted thereto. The roller cone bit body 215 is commonly made from a plurality of legs, in this example three, that are welded together.
FIG. 3 illustrates a cross-section of one leg 301 of a prior art roller cone bit. The joining of the legs forms a fluid inlet 308 and an internal plenum 304. At least one fluid orifice 307 is typically machined in leg 301. The fluid orifice 307 comprises an entrance 302, a nozzle seat 306, an O-ring gland 310, and a receptacle 311 designed for the attachment of a nozzle 313. During drilling, fluid, not shown, enters the bit body 215 at the fluid inlet 308 and continues into the fluid plenum 304. The fluid is forced against a bottom of the fluid plenum 305 until it reaches the fluid orifice 307 where it exits the bit body 215 through the nozzle 313.
Drill bits of the second category are commonly known as “fixed cutter” or “drag” bits. Bits of this type usually include a bit body formed from steel or a matrix material upon which a plurality of cutting elements is disposed. Most commonly, the cutting elements disposed about the drag bit are manufactured of cylindrical or disc-shaped materials known as polycrystalline diamond compact, or PDC. Polycrystalline diamond compact cutters are of extraordinary hardness and drill through the earth by scraping away the formation rather than pulverizing it. For this reason, fixed cutter and drag bits are often referred to as “PDC” bits. Like their roller-cone counterparts, PDC bits also include an internal plenum through which fluid in the bore of the drillstring is allowed to communicate with a plurality of fluid nozzles.
Referring still to FIG. 3, the fluid velocity within the fluid plenum 304 is relatively low. However, as the fluid moves into the fluid orifice 307, it accelerates due to the reduction of flow area. Significantly, the increased fluid velocity through the fluid orifice 307 can cause internal erosion of the drill bit. Internal erosion in a drill bit may typically be related to four parameters: mud weight, mud abrasiveness, flow velocity, and geometrical discontinuities. Over time, the drilling industry has found the need to increase the flow rates through the drill bits, making internal erosion of the fluid orifices a significant source of concern. A ledge 314 formed between the bottom of fluid plenum 305 and fluid orifice 307 is particularly troublesome in drill bits. High flow rates cause the fluid flow to separate at ledge 314 creating recirculation zones that may have sufficient energy to erode the surrounding metal surface. A “washout” occurs when the erosion progresses such that a hole is formed in the bit body 215 that allows the fluid to bypass the nozzle. The washout results in a loss of pressure in the system and requires pulling the drill bit out of the hole to be replaced. This costs the driller a great deal of time and money.
FIG. 4 illustrates one prior art solution disclosed in U.S. Pat. No. 5,538,093 (the '093 Patent), incorporated by reference herein. In the '093 Patent, a sleeve 409 is welded inside of the fluid orifice 307. Sleeve 409 comprises a smoothly contoured fluid entrance 403 which gradually reduces the flow area in preparation for entrance into a nozzle, not shown, at a nozzle seat 406. Fluid entrance 403 helps to eliminate the separation of the fluid and, therefore, reduce the amount of internal erosion. One drawback of this approach is that the sleeve 409 requires a significant amount of space to be effective. As a result, this approach is only available for the large drill bit sizes (i.e., those bits having diameters greater than 11″).
Small drill bits (i.e., those bits having diameters less than 11″) are typically unable to accommodate sleeves in the fluid orifices because there is not sufficient room in the interior of the bit to accommodate the required large fluid orifice without cutting into the side of the bit or into areas reserved for the bit lubrications system, not shown. FIGS. 5 and 6 illustrate a typical small drill bit. To fit a nozzle, not shown, a fluid orifice 505 is usually drilled into the bit body 508 through a bottom of the fluid plenum 504. A nozzle receptacle 509 is then formed inside fluid orifice 505 for the attachment of the nozzle. The drilling of fluid orifice 505 leaves a ledge 506 formed between the bottom of fluid plenum 504 and fluid orifice 505. Depending on the flow rate and the geometry of the particular drill bit, ledge 506 can cause fluid separation to occur with sufficient energy to erode bit body 508 which can lead to a washout. Manufacturing options to remove ledge 506 are limited due to the limited space and accessibility to ledge 506 by machining tools.
A prior art solution for small drill bits is shown in FIG. 9. A drill is inserted through the fluid orifice 505 to machine a relief region 901 substantially towards the bit body axis, not shown. While such a relief region provides some improvements in the flow, such a region fails to fully solve the erosion problems present at higher flow rates.
What is still needed, therefore, are drill bits and methods for designing and manufacturing drill bits having improved internal flow characteristics.